This invention relates to an apparatus for detecting the change of light intensity. More particularly, it relates to an apparatus for detecting the change of light intensity that is equipped with a light intensity changing means such as an electrooptic crystal that allows the intensity of incident light to vary in accordance with the change in a signal to be measured such as an electric field.
An example of the apparatus for detecting the change of light intensity is an electrooptic (E/O) voltage detector which uses an electrooptic material whose refractive index is changed by the strength of an applied electric field. Conventional versions of this E/O voltage detector are disclosed in U.S. Pat. No. 4,446,425, Japanese Patent Application Unexamined Publication Nos. 300969/1988, 9370/1989, etc.
These voltage detectors have an optical probe that is made of an electrooptic material having a light-reflecting surface on its tip. The light reflected from said reflecting surface is received by a photodetector and subjected to photoelectric conversion. However, the dynamic range of the photodetector is not wide enough to allow a small change in light intensity (voltage change) to be detected with high S/N ratio. To overcome this difficulty, it has been necessary that the electric signal to be measured be turned on and off, and the signal of such a modulation frequency be amplified over a narrow band.
In the inventions described in the prior patents listed above, this need can be easily met if the electric signal to be measured is an output signal from a photodetector, since one only need to turn the incident pulse light on and off. However, the problem is that the device that can be measured (i.e., the source of the electric signal to be measured) is limited to one that generates an electric signal in response to input light.
As an alternative, the electric signal to be measured may be directly turned on and off by means of an electric chopper, but, in this case, the waveform of a signal to be measured will be distorted when it passes through the electric chopper, whereby it becomes difficult to achieve correct waveform measurements.
Considering the measurement of a dc voltage, the intensity of light passing through an E/O probe will change if a voltage is applied to it, so that the measurement of the dc voltage should be made possible by monitoring the quantity of transmitted light with a photodetector. In fact, however, the E/O probe has such a low sensitivity that if the applied voltage is only a few volts, the resulting change in the quantity of light is too small to be detected by an optical power meter having an ordinary dynamic range.
It has generally been proposed that an electric signal of interest be chopped as shown in FIG. 14, with only the modulated component being efficiently detected with a lock-in amplifier. This technique, however, is not effective when a dc voltage is to be measured, since the dc electric signal cannot be chopped. Instead, the input light to the optical probe may be chopped as shown in FIG. 15. However, if the output light is simply subjected to lock-in detection, it is the light intensity itself that is measured as shown in FIG. 15(C), and the change in light intensity resulting from the application of a voltage to the E/O probe cannot be detected.
Further, the light-reflecting surface is formed at the tip of the optical probe and it is practically impossible to achieve 100% reflection by this surface. This inevitably causes part of the incident light to leak toward a source of the signal to be measured. If part of the incident light leaks toward the signal source, the light reflected from the latter will be transmitted again through the light-reflecting surface to enter the optical probe, and detected as part of the output signal. If, even in this case, the reflectance of the signal source is constant, correct measurements can be accomplished by processing the output signal in consideration of said constant reflectance and the transmittance of the light-reflecting surface. However, if the signal source is, for example, an LSI, its reflectance will change in accordance with its circuit pattern and this cannot be easily compensated.